1. Technical Field
The present invention relates to a control apparatus for controlling rotation of joints of a robot, and in particular to a control apparatus for controlling rotation of joints having a structure in which the rotation of an electric motor is slowed down by a speed reducer and transmitted to an arm.
2. Background Art
Recently, a variety of robots are used in a variety of fields. Among them is a robot having a joint that includes an electric motor, a speed reducer that slows down the rotation of an output shaft of the motor, and an arm connected to the speed reducer. In controlling a rotational position of the arm in the joint, the motor is controlled in accordance with the rotational position of the arm, which is detected by an encoder. In this case, one may desire to confirm what value is actually used for the rotational position of the arm with the intervention of the speed reducer. As a method of confirming this, PTL 1, for example, discloses a method for detecting a rotational position of an arm using a potentiometer, in a configuration in which the motor and the arm are connected via a torque interrupter.                [Patent Literature 1] JP-B-4369886        
In the configuration as described above, a control apparatus comprised such as of a microcomputer is required to refer to a motor-side rotational position and an arm-side rotational position at the same time point. For this purpose, one idea may, for example, be that the arm-side potentiometer is replaced by an encoder, as in the motor side, to deal with a rotational position in terms of digital data, and that the rotational position data outputted from the individual encoders are latched by respective latch circuits so that the control apparatus can read the data. Under this idea, what is important is the timing of reading the data of the two latch circuits.
In general, a motor is located closer to a control apparatus relative to an arm. Accordingly, in order that a control apparatus parallelly reads the arm-side and motor-side encoder data, it is appropriate that the arm-side data are transferred to the motor side and then both data are collectively and parallelly read in. This, however, may cause an offset in the detection time of both data, the offset being corresponding to the time taken for the transfer mentioned above. In other words, in a configuration of driving an arm of a robot, for example, via a speed reduction mechanism, the motor-side rotational position has a higher resolution and thus changes faster than the arm-side rotational position. Therefore, the received arm-side rotational position data are older than the motor-side rotation position data. For this reason, the offset in the timing associated with both rotational positions disables correct control of the motor.